Excitation Polarization Sensitivity of Plasmon-Mediated Silver

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Excitation Polarization Sensitivity of Plasmon-Mediated Silver Nanotriangle Growth on a Surface Aniruddha Paul, Bart Kenens, Johan Hofkens, and Hiroshi Uji-i* Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F 3001 Heverlee, Belgium S Supporting Information *

ABSTRACT: In this contribution, we report an effective and relatively simple route to grow triangular flat-top silver nanoparticles (NPs) directly on a solid substrate from smaller NPs through a wet photochemical synthesis. The method consists of fixing small, preformed nanotriangles (NTs) on a substrate and subsequently irradiating them with light in a silver seed solution. Furthermore, the use of linearly polarized light allows for exerting control on the growth direction of the silver nanotriangles on the substrate. Evidence for the role of surface plasmon resonances in governing the growth of the NTs is obtained by employing linear polarized light. Thus, this study demonstrates that lightinduced, directional synthesis of nanoparticles on solid substrates is in reach, which is of utmost importance for plasmonic applications.

1. INTRODUCTION Noble metal anisotropic nanoparticles (NPs) have received a lot of attention recently as they find applications in plasmonic optics,1 surface-enhanced Raman scattering (SERS),2−4 surfaceenhanced fluorescence,5 bio(chemical) sensing,6 and metamaterials.7 Silver nanoparticles are themselves a distinctive class of anisotropic nanomaterials owing to their unique size and shapetunable optical and optoelectronic properties.8 Recent advances in the synthesis of anisotropic silver NPs has led to different routes showing excellent size and shape control over the final reaction product.9 The successful application of NPs in optoelectronic and other nanoscale devices imposes stringent conditions on the NPs, for example, monodispersity both in size and shape, stability in ambient conditions, a crystalline morphology with well-defined edges and smooth surface, and so on. In general, the synthesis of silver NPs through the socalled bottom-up approach or wet-chemical route yields particles with good crystalline morphology and well-controlled shapes,9 whereas the top-down lithographic techniques allow for the successful engineering of ordered arrays of prototype nanostructures on solid substrates.1a,b,10 The former approach implies the underlying challenge of the controlled deposition and immobilization of the particles on a solid surface which is of major concern for potential applications. On the other hand, the top-down approach suffers from a poor crystalline morphology and surface roughness as well as from reduced stability under ambient conditions of the obtained metal nanostructures. This compromises the efficiency of devices due to large losses of surface plasmons generated on these structures.11 Therefore, an alternative approach that combines advantages of both bottem-up and top-down routes might be the controlled synthesis of NPs of any desired size and shape directly on a solid substrate by chemical or photochemical © XXXX American Chemical Society

ways. By doing so, ideally the fabricated nanostructures will possess an adequate stability as well as the required morphology. So far little effort has been exerted in this direction.12 There have been a few attempts to grow surfaceimmobilized anisotropic gold NPs, especially nanorods and nanoplates through wet-chemical route, but there are still the issues, for example, polydispersity, that have to be solved yet. Herein, we report an effective and relatively simple route to grow triangular flat-top silver NPs (designated as silver nanotriangles or NTs) directly on a solid substrate from small NPs through a photochemical synthetic route. The photoinduced conversion of small silver seeds (∼5 nm in diameter) into large NPs of various shapes has been reported in solution.13,14 The best studied system in terms of mechanistic detail of the growth process comprises NTs.13c,d The silver NTs show very distinctive size-dependent extinction spectra in the UV−vis range and it has been shown that the progressive red shift of the applied photoexcitation wavelength yields increasingly larger NTs with more red-shifted extinction spectra. Apparently, the surface plasmon resonances of the nanoparticles play a crucial role in the growth process, and the photoexcitation of the dipolar plasmon mode by visible light of appropriate wavelength has been invoked to explain the growth of small NTs into the larger ones.13b,15 However, since in solution an isotropic environment always prevails, we reason that perhaps the best way to investigate the possible role of the surface plasmon resonance is to study the growth process of anisotropic particles on a solid surface in a controlled fashion. Special Issue: Colloidal Nanoplasmonics Received: February 5, 2012 Revised: March 23, 2012

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Thus, our present study focusing on photoinduced growth of nanoparticles on solid substrate has twofold objectives: first, to understand the chemical aspects of the growth process from mechanistic viewpoint; and, second, to examine the role of photoexcited plasmon resonances in the aforementioned growth process.

Scheme 1. Schematic Representation of the Light-Induced Nanoparticle Growth on a Surfacea

2. MATERIALS AND METHODS AgNO3, bis(p-sulfonatophenyl)phenylphosphine, dipotassium salt (BSPP), polyvinylpyrrolidone (PVP), and sodium citrate were purchased from Sigma-Aldrich and used without further purification. All the solutions were prepared in Milli-Q water (18 MΩ, Millipore). The photoinduced NT growth was conducted using the 488 nm line (50 mW) from an Ar+ laser (Stabilite 2017, Spectra-Physics) and the 632.8 nm (35 mW) output from a He−Ne laser (model 1145P, JDS Uniphase Co.). The polarization was tuned using λ/2 and/or λ/4 waveplates for the photoreaction. An aqueous solution of small Ag seed particles (seed solution) was prepared by reducing a solution of AgNO3 with NaBH4 in presence of sodium citrate and a surfactant, cf. BSPP, or a polymer, cf. PVP, following literature procedures, and it was then aged for 8−12 h before proceeding to the next step. Nanotriangles having an average size of 40−45 nm (NT488) were synthesized from a silver seed solution at pH ∼ 11 via 488 nm laser illumination for several hours (typically 14−16 h).13 The NT488s were deposited on a clean glass coverslip in a controlled fashion by adjusting the particle concentration in the colloidal solution. Prior to the deposition, the glass coverslip was treated with 3-aminopropyltrimethoxysilane (APTMS) to immobilize the particles onto the surface. Next, the coverslip decorated with NT488s was rinsed thoroughly with water and immediately immersed in a freshly prepared and adequately aged Ag seed solution without drying the sample. This system was then irradiated with circular or linear polarized 632.8 nm laser light for several hours (typically 24−28 h). After the required hours of illumination, the coverslip was rinsed with Milli-Q water and blow-dried in Ar gas flow. Several topographic and phase images of each sample were recorded with atomic force microscopy in noncontact AC mode (CombiScopeTM 1000 or SmartSPMTM 1000, AIST-NT). The resolution of the AFM setup is typically ±5 nm (in xy-plane) in NP size measurements and less than 0.5 nm when the thickness (z-direction) of the NPs was measured.

a

NT488s were immobilized on a glass surface prior to the immersion of the coverslip in aqueous Ag seed solution (referred to as growth solution). Here the excitation with linearly polarized light is shown, although the excitation with circular-polarized light is similar. Inset: The height or the trigonal axis length of a NT, “d”, as defined in the text.

the values of d were collected from AFM images of multiple samples. In this first set of experiments, bis(p-sulfonatophenyl)phenylphosphine (BSPP) was used as a surfactant both for the Ag seed NPs and for the NT488s. By illumination with circular polarized 632.8 nm light, the NT488s were converted to larger nanoplates directly on the surface of a cover slide as shown in an AFM image in Figure 1a. The shapes of the nanoplates are relatively polydispersed, including triangular, trapezoid, or hexagonal shapes, though the majority of the nanoplates are still triangles (designated as NT633). Large nanoparticles without any specific shapes were also found on the surface. The size distribution of the NT633s on the surface is shown in a histogram in Figure 1b. It can be seen that these NT633s are roughly in the range of 90−100 nm in d-value, which is appreciably larger than the NT488s. The extinction spectrum of the Ag seed solution employed as growth solution was recorded just after the photoreaction. It was found to be almost unchanged as compared to the spectrum before the reaction (Figure 1c). If the NT633s are generated in solution and if they would adsorb on the glass surface, the extinction around 800 nm in the suspension should be much higher. Indeed, when the reaction would occur in solution, then the extinction around 400 nm (from the seed particles) should become lower and the extinction originating from triangles would grow at longer wavelengths and become higher than the extinction at 400 nm. In this experiment, a slight increase of the extinction at 800 nm was found, which is likely originating from nanotriangles detaching from the surface. The extinction meausurments eliminate the possibility that the observed nanotriangles on the surface result from photoinduced growth of Ag seeds present in the growth solution and ensures that the larger NTs (NT633s) grow from NT488s on the surface. In order to obtain a better understanding about the underlying mechanism of the photoinduced growth process, we have performed following control experiments. First, we

3. RESULTS AND DISCUSSION 3.1. Light-Induced Growth on a Surface. To achieve photoinduced formation of silver NTs on a surface, we have designed an experiment that is illustrated in Scheme 1. In typical experiments, we first synthesized small silver NTs in solution via photoirradiation of the seed Ag NPs (∼5 nm diameter) with 488 nm light. The colloidal solution of these small NTs (designated as NT488) shows a characteristic extinction spectrum centered around 500 nm (Figure S1, Supporting Information) with average size of the NTs in the range of 40−45 nm.13d In the next step, these NT488s were deposited on an amino-functionalized glass surface, avoiding particle aggregation by adjusting the concentration of the colloidal solution. The coverslip decorated with NT488 particles was then washed thoroughly with Milli-Q water and immediately immersed in a freshly prepared and adequately aged Ag seed solution (referred to as the growth solution) without drying or exporsing the sample to air. This system was then irradiated with 632.8 nm laser light for several hours. An excitation wavelength longer than 600 nm was specifically chosen to avoid direct photoexcitation of the 5 nm Ag seed NPs in the growth solution, ensuring that only NTs attached on a surface could grow further. The size of the NTs was determined by the height of the triangle, that is, the length of the trigonal axis (assigned as “d” in the inset to Scheme 1), and B

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once the extinction peak is red-shifted compared to the excitation wavelength of the irradiation.13b Another study focusing on the calculated electric field distribution has suggested that upon dipolar plasmon excitation the resultant field tends to concentrate on the tips of the triangle.8a,15,17 In view of this observation, the photoinduced growth of NTs in solution has been rationalized by invoking the photoexcitation of surface plasmon modes of the nanoparticles. It has been proposed that the dipole plasmon resonance of small NTs (such as NT488s) is the principal governing factor for the electron transfer process in the reduction of Ag(I) to Ag(0) at the vicinity of the NT surface.13b,d The key aspect of light-induced growth of NTs is the assignment of the important role of plasmon excitations/modes on the growth mechanism. Though there exist many indications in solution studies which emphasize the importance of excitation of dipolar plasmon modes, given the isotropic environment of the nanoparticle solution, it is hard to pinpoint the effect of different plasmonic modes on the nanoparticle growth process in solution. Since we are able to grow NTs immobilized on a solid surface, it is possible to investigate anisotropic aspects of the light-induced growth process in a controlled fashion, for example, the influence of the polarization direction of the incident light. To this end, NT488s immobilized on a surface, were irradiated with linearly polarized light from a He−Ne laser (632.8 nm). Growth of NT488s to larger NTs (NT633s) was observed in a similar manner as with the incoherent depolarized light, but this time the sample showed a distinctive orientational bias of the obtained NTs (Figure 2a). The histogram shown in Figure 2c displays a distribution of the angle made by one of the trigonal axis of each NT with respect to the direction of the polarization of the excitation light during the NTs light-induced growth. Note that the angular distribution that is covered spans |θ| ≤ ± 30°. This is due to the trifold symmetry of the triangles, as a rotation by ±30° will bring another trigonal axis of the NT perpendicular to the polarization direction (Figure 2b). It is clear from Figure 2c that the NT633s formed by the linearly polarized light are preferentially oriented in line with the direction of the polarization. In contrast, when the circularpolarized 632.8 nm laser light was used for the light-induced growth, the angular distribution is found to be random with respect to an arbitrary vertical axis, indicating an isotropic orientation of the NT growth on the surface (Figure 2d). The clear-cut difference between NT growth with the linear and circular polarized light provides direct evidence of the crucial role of surface plasmons in governing the growth of the NTs. Since NT488s are immobilized via electrostatic interaction with many amine groups on the surface, it is unlikely that NT488s rotate during the growth process. This suggests that the orientation of NTs would most likely not alter after the growth of a NT488 to a NT633, if the NT633 is still a single crystal. In other words, among the surface-immobilized NT488s, only those which are oriented along the incident light polarization can grow into larger NTs with high efficiency. Thus, the growth of NTs which are not favorably oriented with respect to the polarization of light must be less efficient. This hypothesis is supported by the observation of the Gaussian-like angular distribution of growth direction shown in Figure 2c. To have a better understanding, we have performed numerical simulations using the finite differential time domain (FDTD solution, Lumetical Solution Inc.) method to estimate the electromagnetic field intensity, |E|2, on a NT (d = 40 nm,

Figure 1. (a) AFM images of the NT633s grown on a glass substrate from surface-immobilized NT488 capped with BSPP in BSPP-capped Ag seed solution via irradiation with circular-polarized 632.8 nm laser (scale bar: 200 nm). (b) Histogram of size of NT633s. (c) Extinction spectra of BSPP-capped Ag seed growth solution before and after the photoreaction (dashed and solid line, respectively).

have attempted to carry out this photochemical reaction without using Ag seeds in the growth solution; that is to say, only AgNO3 in solution with appropriate concentrations of sodium citrate and BSPP have been used as growth solution. No further growth of NT488 has been confirmed even after prolonged irradiation of the sample, which indicates the vital role of the Ag seed particles in this reaction. Second, we have repeated this photoreaction by using a geminate Ag seed solution without appreciable aging. In this case, only growth into particles of nonspecific shapes occurred instead of flat NTs with well-defined shape. Perhaps the excess amount of reducing agent (NaBH4) present in the unaged seed solution perturbs the required conditions for the photoinduced growth leading to nanotriangles. Third, the surfactant (the capping argent, such as BSPP and PVP) is reported to be one of the key factors for the photochemical preparation of NPs13c,16 We indeed confirm that the surfactant apparently plays an active role in the reaction course and that it strongly affects the polydispersity of the final product (see the Supporting Information). 3.2. Effect of Light Polarization during the Photoinduced Growth. Since we demonstrated that the observed growth of NPs on the surface is truly a photoinduced process, a closer inspection of the influence of effect of the light polarization on NP growth seemed interesting. Before going to the detailed discussion, we will briefly summarize some observations and studies reported in literature. The extinction spectrum of a nanotriangle of a given size is characterized by an intense narrow band ranging from visible to near-infrared wavelengths. This band is attributed to the in-plane dipolar plasmon excitation.15 With an increasing size of the Ag NTs, the plasmon band shifts progressively to longer wavelength. In the case of a photoinduced reaction, NTs do not grow further C

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Figure 2. (a) Representative AFM image for a given sample where the photoinduced growth of surface-immobilized NT488s was carried out with linearly polarized 632.8 nm laser light. (b) Definition of the angle θ, describing the angle between the polarization direction of incident light and the nearest perpendicular bisector (trigonal axis) of the triangle (left). Scheme illustrating the fact that rotation by ±30 will bring one of its trigonal axis perpendicular to the line of polarization, thereby making the triangle effectively perpendicular with respect the incident polarization (right). (c) Angular distribution of the NTs grown on the surface by 632.8 nm laser light having linearly polarized output (top panel) and circular polarized output (bottom panel).

Figure 3. (a) FDTD simulations of electric field enhancement contours for a silver NT (d = 40 nm) with different polarization angles of the incident light (632.8 nm). The direction of the polarization is indicated with a white double-headed arrow in each image. The nearest trigonal axes of the NT are at (i) 0°, (ii) 10°, (iii) 20°, and (iv) 30°. (b) Maximum electric field intensity at the apex “x” (square with a black line), at “y” (circle with a red line), and at “z” (triangle with a blue line) as a function of the polarization angle of the incident light.

corresponding to NT488) upon irradiation with linear polarized 632.8 nm light. In the case that one of the trigonal axes of the NT is parallel to the direction of the light polarization (case (i) in Figure 3a), the photoexcitation of the surface plasmons results in an accumulation of the electric field (E-field) at the tips of the triangle with a symmetric distribution with respect to the given trigonal axis. The E-field is mostly enhanced at one of the tips, indicated by “x” in case (i) of Figure 3a, while comparatively less but substantial enhancement occurs at the other two tips as well. From the experimental observation, we conclude that this plasmonic excitation triggers the facile photochemical growth of surface-bound NT488s toward larger NTs. In contrast, when the NTs have one of their edges parallel to the polarization direction of the incident light, the dipolar plasmon excitation is manifested by an E-field distribution confined in the corresponding edge and centering around the two adjacent tips (Figure 3a, case iv). If the light-induced growth is due to the plasmons field enhancement, this orientation should be the least probable to grow into larger NTs as experimentally observed in Figure 2c. Thus, when the trigonal axis of particles is oriented at an unfavorable angle with respect to the incident polarization (cases ii and iii in Figure 3a), the particles are not the best candidates for the photoselection, thereby becoming less susceptible to growth into proper triangular particles. For example, for case (ii) in Figure 3a where the polarization is at 10° offset with respect to the trigonal axis, the surface plasmon localization is unsymmetrical with respect to the axis and the Efield intensity is low at one tip of the triangle. Our model of

light induced NT growh predicts that such orientation should have less probability to grow larger NTs, and this is evident from the Gaussian distribution shown in Figure 2c. In the case of other larger particles, such as tub-shaped, hexagonal particles or others, multipolar (quadrupolar and/or higher) plasmon modes may also play an important role in the growth process. In this context, it is to be noted that some simulations suggested that in-plane quadrupolar resonance leads to electric field distribution at the edges of a triangle.8a,15,17 Therefore, it has been presumed in solution studies that role of quadrupolar resonance is to assist the prism fusion process leading to very large NTs and other anisotropic particles such as tub-shaped particles, rhombus, and so forth. For example, fusion of two NTs can result in a rhombus, while fusion of three NTs leads to a tub-shape, and four to a rhombus or a large triangle.13b However, in the present study, we have also observed a fare share of irregular anisotropic particles grown on the solid surface (Figures 1 and 2a). Since the particles are surface-immobilized, a “prism fusion” type phenomenon is very unlikely in the present case. Although a small fraction of detached NT488s could exist in the growth solution and they could grow into NT633s (seen as a small extinction in the spectrum of Figure 1c), the fraction should be too low to result in “prism fusion”. The origin of the irregular anisotropic particle growth is the subject of a future study. D

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(4) (a) Hutchison, J. A.; Centeno, S. P.; Odaka, H.; Fukumura, H.; Hofkens, J.; Uji-i, H. Subdiffraction Limited, Remote Excitation of Surface Enhanced Raman Scattering. Nano Lett. 2009, 9, 995−1001. (b) Laurier, K. G. M.; Poets, M.; Vermoortele, F.; De Cremer, G.; Martens, J. A.; Uji-i, H.; De Vos, D. E.; Hofkens, J.; Roeffaers, M. B. J. Photocatalytic growth of dendritic silver nanostructures as SERS substrates. Chem. Commun. 2012, 48, 1559−1561. (c) Lu, G.; Li, H.; Zhang, H. Nanoparticle-coated PDMS elastomers for enhancement of Raman scattering. Chem. Commun. 2011, 47, 8560−8562. (5) (a) Lakowicz, J. R.; Ray, K.; Chowdhury, M.; Szmacinski, H.; Fu, Y.; Zhang, J.; Nowaczyk, K. Plasmon-controlled fluorescence: a new paradigm in fluorescence spectroscopy. Analyst 2008, 133, 1308− 1346. (b) Enderlein, J. Spectral properties of a fluorescing molecule within a spherical metallic nanocavity. Phys. Chem. Chem. Phys. 2002, 4, 2780−2786. (c) Faessler, V.; Hrelescu, C.; Lutich, A. A.; Osinkina, L.; Mayilo, S.; Jäckel, F.; Feldmann, J. Accelerating fluorescence resonance energy transfer with plasmonic nanoresonators. Chem. Phys. Lett. 2011, 508, 67−70. (d) Maye, M. M.; Gang, O.; Cotlet, M. Photoluminescence enhancement in CdSe/ZnS-DNA linked-Au nanoparticle heterodimers probed by single molecule spectroscopy. Chem. Commun. 2010, 46, 6111−6113. (e) Lin, H.; Centeno, S. P.; Su, L.; Kenens, B.; Rocha, S.; Sliwa, M.; Hofkens, J.; Uji-i, H. Mapping of Surface-Enhanced Fluorescence on Metal Nanoparticles using SuperResolution Photoactivation Localization Microscopy. ChemPhysChem 2012, 13, 973−981. (6) (a) Homola, J. Surface plasmon resonance sensors for detection of chemical and biological species. Chem. Rev. 2008, 108, 462−493. (b) Sepulveda, B.; Angelome, P. C.; Lechuga, L. M.; Liz-Marzan, L. M. LSPR-based nanobiosensors. Nano Today 2009, 4, 244−251. (7) (a) Shalaev, V. M.; Cai, W. S.; Chettiar, U. K.; Yuan, H. K.; Sarychev, A. K.; Drachev, V. P.; Kildishev, A. V. Negative index of refraction in optical metamaterials. Opt. Lett. 2005, 30, 3356−3358. (b) Grigorenko, A. N.; Geim, A. K.; Gleeson, H. F.; Zhang, Y.; Firsov, A. A.; Khrushchev, I. Y.; Petrovic, J. Nanofabricated media with negative permeability at visible frequencies. Nature 2005, 438, 335− 338. (8) (a) Hao, E.; Schatz, G. C. Electromagnetic fields around silver nanoparticles and dimers. J. Chem. Phys. 2004, 120, 357−366. (b) Jain, P. K.; El-Sayed, M. A. Plasmonic coupling in noble metal nanostructures. Chem. Phys. Lett. 2010, 487, 153−164. (c) Halas, N. J.; Lal, S.; Chang, W. S.; Link, S.; Nordlander, P. Plasmons in Strongly Coupled Metallic Nanostructures. Chem. Rev. 2011, 111, 3913−3961. (d) Sau, T. K.; Rogach, A. L.; Jackel, F.; Klar, T. A.; Feldmann, J. Properties and Applications of Colloidal Nonspherical Noble Metal Nanoparticles. Adv. Mater. 2010, 22, 1805−1825. (9) (a) Xia, Y. N.; Sun, Y. G. Shape-controlled synthesis of gold and silver nanoparticles. Science 2002, 298, 2176−2179. (b) Grzelczak, M.; Perez-Juste, J.; Mulvaney, P.; Liz-Marzan, L. M. Shape control in gold nanoparticle synthesis. Chem. Soc. Rev. 2008, 37, 1783−1791. (10) Ueno, K.; Juodkazis, S.; Mizeikis, V.; Sasaki, K.; Misawa, H. Clusters of closely spaced gold nanoparticles as a source of two-photon photoluminescence at visible wavelengths. Adv. Mater. 2008, 20, 26− 30. (11) (a) Kusar, P.; Gruber, C.; Hohenau, A.; Krenn, J. R. Measurement and Reduction of Damping in Plasmonic Nanowires. Nano Lett. 2012, 12, 661−665. (b) Tinguely, J.-C.; Sow, I.; Leiner, C.; Grand, J.; Hohenau, A.; Felidj, N.; Aubard, J.; Krenn, J. Gold Nanoparticles for Plasmonic Biosensing: The Role of Metal Crystallinity and Nanoscale Roughness. BioNanoScience 2011, 1, 128−135. (c) Chen, Y.; Munechika, K.; Ginger, D. S. Dependence of fluorescence intensity on the spectral overlap between fluorophores and plasmon resonant single silver nanoparticles. Nano Lett. 2007, 7, 690−696. (12) (a) Wei, Z. Q.; Mieszawska, A. J.; Zamborini, F. P. Synthesis and manipulation of high aspect ratio gold nanorods grown directly on surfaces. Langmuir 2004, 20, 4322−4326. (b) Sajanlal, P. R.; Pradeep, T. Electric-field-assisted growth of highly uniform and oriented gold nanotriangles on conducting glass substrates. Adv. Mater. 2008, 20, 980−983. (c) Beeram, S. R.; Zamborini, F. P. Purification of Gold

4. CONCLUSION In conclusion, we have established an effective route to photoinduced growth of NTs by immobilizing small Ag NTs on a glass substrate and by irradiating them with light of an appropriate wavelength. At the same time, more insight into the mechanistic aspects of this photochemical growth process is obtained from this study. We have provided evidence for the control exerted by the surface plasmon resonances in governing the growth of the NTs by employing different polarization of light. Thus, this study suggests the possibility of light-induced, directional synthesis of nanoparticles on a solid substrate, which is of utmost importance for plasmonic applications. This synthesis protocol has great potential to develop stable plasmonic materials with a designed array or pattern of nanoparticles for various applications ranging from subwavelength optics to sensing of biomolecules.

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ASSOCIATED CONTENT

S Supporting Information *

Extinction spectrum of NT488s and the surfactant dependence on the NT synthesis on a surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support of the “Fonds voor Wetenschappelijk Onderzoek FWO” (Grants G.0459.10, G.0402.09, G0413.10, G0181.10, G0697.11, G0197.11, G0259.12), the K.U.Leuven Research Fund (GOA 2011/03, CREA2009), the Federal Science Policy of Belgium (IAP-VI/27), the Hercules foundation (Grant HER/08/21), and the Flemish government (Long term structural funding, Methusalem funding CASAS METH/08/04 and the EC Marie-Curie ITN-SUPERIOR (PITN-GA-2009-238177) is gratefully acknowledged. This work was supported by the JST PRESTO program and an ERC starting grant (Grant # 280064) awarded to H.U.

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REFERENCES

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dx.doi.org/10.1021/la300533h | Langmuir XXXX, XXX, XXX−XXX